Farrell or her people were irked that her voice had been used earlier in the season without the proper hoops being jumped through, or something like that, so they didn't allow flashbacks of her. At least, that's my understanding of the situation.
I've read interviews where she said that she didn't "quit" DS9, her contract was only for 6 seasons. I heard rumors of money issues being the reason she didn't renew for another year.
Which brings up another question...did the other actors originally sign up for 7 years and if so, why was her contract only for 6?